Niels Hoegel was convicted of two murders in 2015 but investigators believe he killed at least 90 patients with overdoses of heart medication.

File photo from February 2015 shows former nurse Niels Hoegel, accused of multiple murders and attempted murders of patients, covering his face with a file at the district court in Oldenburg, Germany. Hoegel was convicted in 2015 of two murders and two attempted murders. (AP)

A male nurse jailed for life two years ago, for killing two hospital patients with lethal drug overdoses, murdered at least 90 patients in total and possibly twice as many, police said on Monday, calling it post-war Germany’s worst killing spree.

Niels Hoegel, 40, was jailed in February 2015 for two murders and four counts of attempted murder or causing bodily harm on intensive care patients at the Delmenhorst hospital near the northern city of Bremen.

Police said forensics experts had since exhumed and analysed more than 130 additional bodies and found evidence of a vastly higher death toll at two hospitals where Hoegel had worked between 1999 and 2005.

“The insights we were able to gain are terrifying, they surpass what we could have imagined,” said Johann Kuehme, police chief in the city of Oldenburg, where the other hospital is located.

The death toll “is unique in the history of the German republic”, said the chief police investigator in the case, Arne Schmidt, adding that Hoegel killed “without a discernable pattern” and preyed especially on those in critical condition.

There was “evidence for at least 90 murders, and at least as many (suspected) cases again that can no longer be proven”, he told a news conference, declaring himself “speechless” at the outcome.

Hoegel has admitted to injecting patients with drugs that can cause heart failure or circulatory collapse so he could then try to revive them and, when successful, shine as a saviour before his medical peers.

He earlier testified that at times he acted out of “boredom”, that he felt euphoric when he managed to bring a patient back to life, and devastated when he failed.

Murderous obsession

After the revelations, police and prosecutors three years ago launched a special forensic commission dubbed “Kardio” (Cardio) to look into other patient deaths.

Presenting their findings, police said on Monday that 134 bodies had been exhumed and tested for traces of the deadly drugs, and that police also reviewed scores of medical records and questioned hundreds of witnesses.

The cause of death in many cases could not be determined because the bodily remains had been cremated, said Kuehme.

The grisly revelations date back to June 2005, when a female nurse witnessed Hoegel trying to inject a patient at the Delmenhorst hospital.

The patient survived and Hoegel was arrested. In June 2008, he was sentenced to seven-and-a-half years in jail for several cases of attempted murder.

Amid the media publicity, a woman then contacted police, voicing suspicion that her deceased mother had fallen victim to the killer nurse. The authorities exhumed several patients’ bodies and detected traces of the drug in five of them, declaring it either the definitive or possible contributing cause.

Hoegel was jailed for life in 2015, but at the time it was clear he had murdered many more patients, with investigators admitting they may never know the true number.

Several senior medical staff at Delmenhorst also face separate trials for having failed to act speedily over the high number of suspicious deaths when Hoegel was on duty, while investigations continue at the Oldenburg clinic.